Many modern telephones are controlled by microprocessors. Some types of microprocessors can be designed to consume very little power, and thereby enable telephones to obtain their operating power from the telephone line itself, rather than from an electrical outlet. Such telephones are called "line-powered" telephones.
Line-powered telephones can be divided into two classes, based on their power consumption when inactive. The first class includes those which draw a small current from the telephone line, called a leakage current. This current is not sufficient to power all features of the telephone, but is sufficient to maintain the microprocessor in an alert state.
The alert microprocessor monitors the keypad of the telephone. When key presses are detected, the microprocessor infers that a telephone call is impending, and connects the telephone to the telephone line, to supply full power. The telephone becomes active.
The second class includes telephones which draw no power at all from the telephone line when inactive. In this class of telephones, unlike the first class, key presses are not detected when the telephone is inactive, because the microprocessor is unpowered at that time. To allow detection of key presses when the telephone is inactive, such a telephone may be equipped with a storage battery may supply power. However, maintaining such a battery is seen as complex and expensive.